A parking lot guidance method is known which include performing a white line recognition process on the image data, obtained by an onboard camera imaging a parking area, and recognizing a parking frame on the basis of the recognition result (see JP2007-315956A, for example).
In the parking lot guidance method as described in JP2007-315956A, when a white line representing a parking frame cannot be recognized for some reasons, such as that no white line exists or the white line is unclear, the parking frame cannot be recognized and an available parking space cannot be estimated.